A typical and very important use for abrasion resistant ferrous alloys of the general type here under consideration is for sealing rings in mud seal assemblies of earth moving machines and the like. Such an assembly, as exemplified by U.S. Pat. No. 3,940,154, comprises a pair of coaxial metal sealing rings which rotate relative to one another and which have axially opposing annular sealing surfaces that are lapped to accurate flatness and are engaged with one another under bias. The lapped surfaces of the sealing rings are maintained in sealing engagement by elastomeric rings, one for each of the metal rings, whereby the metal rings are held in coaxial relation to one another and to other parts. Each of the elastomeric rings also serves to transfer torque to its metal ring, so that relative rotation occurs only at the seal between the engaged annular surfaces of the metal rings and not between the metal rings and the elastomeric rings or between the latter and their holders.
The metal sealing rings in such a seal assembly must be extremely hard to resist abrasion under the friction which they impose upon one another by their relative rotation. They must also resist surface galling and should not have a tendency to seize after extended periods of rest or non-use. At least their rotatably engaged sealing surfaces must also be resistant to corrosion in the presence of salt water and the like. Furthermore, they must retain their significant physical properties notwithstanding subjection to mechanical shocks and other adverse conditions including extremes of environmental temperatures ranging from arctic conditions on the order of minus 40.degree. F. to desert temperatures on the order of 120.degree. F.
One abrasion resistant alloy heretofore used for mechanical sealing rings in mud seal applications was commercially known as "Haynes 93". It was highly satisfactory from the standpoint of its ability to resist wear in a gritty environment, but it was also extremely expensive in that its composition, as set forth in U.S. Pat. No. 4,094,514, included about 6.5% cobalt, 16% molybdenum, 17% chromium and a little under 2% of vanadium. While all of these alloying materials are more or less expensive, molybdenum and cobalt are particularly critical. About 95% of the cobalt used in the United States is imported, and the quantities available here are so limited that users are sold only restricted allocations of it. Although the major suppliers of molybdenum are in the United States, world-wide demand exceeds the available supply, and molybdenum, too, is subject to allocations. There have been times in the recent past when allocations of these alloying metals have restricted the number of castings that could be produced.
In a sense, "Haynes 93" was better than necessary because sealing rings made from that alloy outlasted the bearings that they protected. Since the sealing assembly must be discarded when bearings are replaced, the use of "Haynes 93" for mud seal sealing rings represented a certain amount of waste of critically scarce alloy metals.
A somewhat less expensive abrasion resistant alloy, commercially designated "Haynes 589", was disclosed in U.S. Pat. No. 3,067,026. "Haynes 589" required an acceptably low percentage of cobalt, but it contained an undesirably high 12% to 20% of almost equally critical molybdenum. "Haynes 589" was particularly intended for use in nuclear reactors, but it found some application in seal rings, although it could not be used for all seal ring applications because it shows less wear resistance than "Haynes 93".
An abrasion resistant alloy less expensive than "Haynes 589", disclosed in U.S. Pat. No. 4,094,514 (applied for in 1977) is commercially known as "CR-19". Less molybdenum is needed for "CR-19" than for either "Haynes 93" or "Haynes 589". "CR-19" includes 0.25% to 1.25% of critical cobalt; and this compares favorably with the 5.5% to 7% of cobalt specified for "Haynes 93". However, "CR-19" also requires from 1.75% to 3% of tungsten, which is expensive and which tends to give rise to foundry problems when incorporated into alloys of the type here under consideration because it causes the molten metal to be relatively sluggish.
"CR-19" is slightly less wear resistant than "Haynes 589". Because of this comparative lack of wear resistance, the use of "CR-19" in mud seals has heretofore been confined to inexpensive short-life seals for specific applications. "CR-19" is not regarded as suitable for final drive seals nor for mud seals where long seal life is required. Mud seals are those which are directly adjacent to endless treads or are in similar locations where they are constantly exposed to grit; whereas final drive seals are located in drive hubs where they are better protected from abrasive foreign matter but are still exposed to some grit. Typically, seal rings for mud seal assemblies are not over about 6 in. (150 mm.) in diameter, while final drive seal rings have diameters of about 12 in. (300 mm.) and upwards.
It will be apparent that there has been an urgent need for an abrasion resistant alloy which requires less of the critical and expensive alloying materials--particularly cobalt and molybdenum--than "Haynes 93" or "Haynes 589" but which is nevertheless sufficiently more wear resistant than "Haynes 589" to be suitable for use in long-life sealing rings of mud seal assemblies. To be suitable for all categories of sealing rings, such an alloy does not have to be quite as wear resistant as "Haynes 93", but its cost and its content of critical alloy metals should compare favorably with those of "Haynes 589" and "CR-19".
The art has thus been confronted for a number of years with an urgent need for an abrasion resistant alloy having a combination of physical and economic characteristics that suit it for all categories of metal sealing rings and containing small enough percentages of cobalt and molybdenum to effectively extend the critically small supply of those alloying metals. This problem has had no solution that was obvious to those skilled in the art, as is apparent from the fact that production of alloys suitable for grit seal rings has for some time been limited by the availability of the critical metals and has been insufficient to satisfy demands. Having in mind that, among other uses, such grit seal rings can find application in mechanized military equipment, it can be appreciated that there has been an active search for a solution to the problem.